The fluctuations of the propeller load of a high speed container carrier oscillating on the swells have begun to come into question as it's speed becomes higher and higher. The measured results of model experiments in the self-propulsion test in oblique regular waves have shown the fluctuations that amount to be 50% of mean value, at the wave height 1/40 of model length. These phenomena of remarkably great vibrating bearing propeller forces have already been pointed out by Hoshino et al., Schwanecke, Vedeler and Lipis. In the present work, the author attempted to estimate the fluctuations of propeller load applying the unsteady aerofoil theory and the theoretical calculations of ship motions based on the New Strip Method in oblique regular waves. The fluctuations of propeller load were shown to consist of three harmonics for one blade, namely encounter circular frequency ω_e, the frequency compounded by circular frequency of propeller revolution ω_e±Ω. For the total blades only one harmonic ω_e was shown to act. Predominant components in the amplitude of these fluctuations have been made clear that wave orbital motion contributes exceedingly, and then in the order of surging and pitching motions of a vessel. Theoretical calculations by the present method have been compared with the above mentioned model experiments, and shown to be in good agreement with them, except the region of short wave length where the fluctuations of fluid velocity are affected by the deformation of waves due to the presence of the vessel. The thrust fluctuations of loads acting on one blade and that for the total blades as a whole propeller were simulated by the computer, the results of which are similar to some experimental results measured by Takaishi and others.